This invention relates to novel designs for the optical system used in liquid crystal projectors. In particular, these designs relate to projectors that make use of reflective liquid crystal light valves. There are two major arrangements to be disclosed. In one arrangement, three reflective liquid crystal panels are used for full color projection displays. In the other arrangement, two liquid crystal panels are use for full color projection. In the latter case, one of the panels is used for green light and the other panel is used to modulate both blue and red lights.
Liquid crystal display projectors are normally based on transmittive type panels. Such liquid crystal projectors are produced commercially and many standard designs for their optical arrangements are in use. With the introduction of reflective liquid crystal light valves, the projection optics inside the projectors has to be totally redesigned. This is particularly true for full color projection displays utilizing silicon backplane microdisplays. Of particular interest are designs that are compact, high throughput, small F-number, high contrast and high brightness. For compactness, it is necessary for the color separation and color recombination to be carried out in the same set of optical elements.
The optical system of a full color reflective mode LC light valves projection, must have following characteristics: (1) Large output light flux, which means large system optical invariance, or system etandue with the LC light valves. (2) Dual-polarization utilization. (3) High efficiency color separation and recombination which is polarization independent. (4) Compact and small retrofocus for the projection lens. The color separation and recombination prisms disclosed in the prior art do not meet these requirements. It is the purpose of the present invention to disclose a color splitting/recombining prism which is of high efficiency and compact and easy to align and manufacture.
In the conventional reflective projector system, the collimated light source is first split into three primary colors by two dichroic color filters (usually with a first blue filter and a second red filter). Then these light beams are directed onto the corresponding light valves along different optical paths. The reflected light beams, having changed polarizations, are then recombined using two dichroic filters. These filters can be the same set of color separating filters or different ones. The reflected light is separated from the incident light using a polarizing beam splitter (PBS) and finally projected onto the screen. U.S. Pat. No. 4,687,301 discloses one such color separation-recombining optical assembly. Dichroic filters immersed in index matching fluid are used for both color separation and recombination. The angle of incidence on the blue filter is 24xc2x0 while it is 12xc2x0 on the red filter. At an incidence angle of 24xc2x0, there is considerable s-p polarization shift of the band edge of the dichroic filters.
U.S. Pat. No. 4,969,730 describes a three-prism assembly which is commonly known as a color splitting prism. This prism acts as both a color separator and a color recombiner. It is in principle the same as the invention disclosed in U.S. Pat. No. 4,687,301, but much improved in terms of ease of fabrication. The blue filter and red filter are coated onto the surfaces of the prisms. The angles of incidence are all 30xc2x0. A PBS is also used to separate the incident beam from the polarization modulated reflected beam. At the incidence angle of 30xc2x0, however, the problem of s-p polarization shift is greater.
U.S. Pat. No. 5,644,432 describes a projection system where the color separator and recombiner consist of the same three-prism assembly. A PBS is used to separate the incident and reflected light beams. In this case, there is no air gap in the blue filter so that the 3 prisms can be glued together. The blue and red dichroic filters have large angles of incident of 30xc2x0 in order to maintain a short back working distance for the projective lens.
In U.S. Pat. No. 5,658,060, Dove et al described a prism assembly whereby color separation and recombination can be carried out using two coatings with the assistance of total internal reflections. Kwok et al (H. S. Kwok et el, Applied Optics, Vol.39, pp168-172, 2000) described a significant improvement of that prism assembly by reducing the s-p polarization splitting of the dichroic coatings. This was accomplished by reducing the angle of incidence on all the dichroic coatings from 24xc2x0 to 16xc2x0. The three-prism designs are called trichroic prism assemblies.
While the s-p polarization shift effect has been greatly reduced, the trichroic prism assembly is quite difficult to assemble. Additionally, air gaps are needed in the assembly of the prisms. Stray reflection inside the prism is quite troublesome. The optical path length is also rather large, as there are quite a few prisms. This makes the back focal length of the projector lens quite long, which results in design complexity and small acceptance angles.
In U.S. Pat. No. 5,953,083, Sharp et al describes a color separation filter stack whereby the polarization direction of a particular color band can be rotated by 90xc2x0, while leaving the rest of the light unchanged in polarization. This color separation filter stack is the basis of a projector with 4 polarizing beam splitters (PBS). In this design, the color separation and color recombination occur at different optical surfaces. Yet the design is quite compact and the back focal length of the projection lens can be quite short, resulting in a good optical design.
The present invention describes further improvement in the optical arrangement of the LCD projector whereby the number of optical elements have been reduced significantly. Moreover, the requirements on the coatings have been reduced so that mass production is possible. In the present invention, the number of PBS in the entire projection system is reduced to one or two. The required back focal length of the projection lens is also very short, being the dimension of one or two PBS.
In the present invention, the color separation filter stack as described by Sharp et al can be utilized to separate out the complementary green and magenta lights. This filter stack is used if the input light is linearly polarized. Alternatively, if the input light is unpolarized, a simpler arrangement of just one PBS can be used, without the need for the color separation filter stack. But the disadvantage is that half of the light will not be utilized in the image formation process. This is often acceptable since the resulting optical arrangement is so simple and can have very small F-numbers that can more than compensate for the brightness of the projector.
One important feature and requirement for the present invention is the entrance PBS that has to be broadband and has to have wide acceptance angle. This kind of PBS is available from many sources and will not be the subject of the present invention.